Question: Which of these numbers is prime? ${3,\ 35,\ 45,\ 91,\ 94}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 35 are 1, 5, 7, and 35. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 91 are 1, 7, 13, and 91. The factors of 94 are 1, 2, 47, and 94. Thus, 3 is a prime number.